President Donald Trump's biggest achievement in his first 100 days? Easy. He stopped the left.

Measure Trump's first 100 days not just by looking at what he has or has not accomplished. Look at what America would have experienced under the alternative: Hillary Clinton.

Under Clinton, the debate would not be on how to replace Obamacare, but how quickly can the left realize its ultimate ambition, a Canadian-style, single-payer system. Under Clinton, the issue would not be how steep the tax cuts, but how many "rich" people, also known as job creators, would experience yet another growth-restricting tax hike.

Under Clinton, the $100 billion-plus annually in new regulations imposed by President Barack Obama — much of it to fight "climate change" — would continue to rise. This has stopped. President Trump signed an executive order that requires an elimination of two regulations for every new regulation proposed by an executive department or agency in 2017, with a zero-dollar net increase in the cost of regulations.

Under Clinton, newly confirmed conservative Justice Neil Gorsuch would've been another Ruth Bader Ginsburg/Sonia Sotomayor/Elena Kagan clone. Four left-wing SCOTUS justices, in the Heller case, ruled that there is not an individual right to keep and bear arms. Spare us a fifth one.

Trump, too, has put the left-wing media on notice. No more Mr. Nice Guy. Through WikiLeaks, we found that John Harwood, a debate moderator, emailed a letter to Hillary Clinton campaign manager John Podesta, bragging about a question he had put to Donald Trump. He also emailed advice on dealing with the challenge posed by Dr. Ben Carson. Staffers for newsmen Jake Tapper and Wolf Blitzer of CNN contacted the Democratic National Committee to seek questions they might put to Republican presidential candidates. There were many other examples of flat-out collusion, well beyond the liberal bias we've come to expect.

President Trump also changed eight years of Obama's "leading from behind" foreign policy by using our largest non-nuclear bomb on ISIS in Afghanistan and bombing Syria for its use of chemical weapons. Under Obama, we pulled out all the troops from Iraq, despite the objections of his foreign policy and national security and defense teams. One of the members of the Joint Chiefs of Staff, now-retired Army Gen. Ray Odierno, said: "I go back to the work we did in 2007 (through) 2010, and we got into a place that was really good. Violence was low, the economy was growing, politics looked like it was heading in the right direction. ... We thought we had it going exactly in the right direction, but now we watch it fall apart. It's frustrating. ... I think, maybe, if we had stayed a little more engaged, I think maybe it might have prevented it."

Under Obama, we bombed Libya, a mission that the Obama administration admitted was done for humanitarian reasons. Libyan dictator Moammar Gadhafi had paid reparations for the terrorist explosion of Pan Am Flight 103 over Lockerbie, Scotland. And following the American-led invasion of Iraq, Gadhafi had surrendered his weapons of mass destruction to the U.S. Yet under President Obama, America joined the French and British in bombing Libya to rid the country of Gadhafi. Incredibly, after criticizing President George W. Bush for not thinking through the invasion of Iraq, when Obama was asked what he thought was the "biggest mistake" of his presidency, Obama said, "Probably failing to plan for the day after what I think was the right thing to do in intervening in Libya."

For the most part, President Trump has delivered on his promises, or has attempted to deliver on them.

Take immigration. While the wall construction has not begun, nor has Trump been able to figure out a way to get Mexico to pay for it, a psychological wall has already gone up. Border-crossing apprehensions were down more than 60 percent from January to March, after Trump made it clear the welcome mat for illegal entry has been rolled up.

As for the so-called "failure" to repeal and replace Obamacare, businesspeople nevertheless have confidence that whatever emerges will be less expensive, less onerous and less intrusive than what they would have faced under a President Hillary Clinton.

Ultimately, Trump will likely be judged on one thing — the economy. If he manages a 4 percent GDP growth, as promised during the campaign, few will care about his nocturnal tweets.

After 100 days of Trump, the earth did not open up and swallow America. His critics can crawl out from under the bed. The stock market has hit record highs; small-business and consumer confidence polls show optimism in levels not seen anywhere from 10 to nearly 40 years, depending upon the poll. Something is happening.

When Ronald Reagan died, George Will wrote: "Today Americans gratefully recall that at a turbulent moment in their national epic, Reagan became the great reassurer, the steadying captain of our clipper ship. He calmed the passengers — and the sea."

It's not too soon to wonder whether someday something similar might be said of President Trump.